One document matched: draft-ietf-httpbis-header-compression-11.xml
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<rfc category="std" ipr="trust200902" docName="draft-ietf-httpbis-header-compression-11">
<front>
<title abbrev="HPACK">HPACK - Header Compression for HTTP/2</title>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google, Inc</organization>
<address>
<email>fenix@google.com</email>
</address>
</author>
<author initials="H." surname="Ruellan" fullname="Hervé Ruellan">
<organization>Canon CRF</organization>
<address>
<email>herve.ruellan@crf.canon.fr</email>
</address>
</author>
<date year="2015"/>
<area>Applications</area>
<workgroup>HTTPbis Working Group</workgroup>
<keyword>HTTP</keyword>
<keyword>Header</keyword>
<abstract>
<t>
This specification defines HPACK, a compression format for
efficiently representing HTTP header fields, to be used in
HTTP/2.
</t>
</abstract>
<note title="Editorial Note (To be removed by RFC Editor)">
<t>
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at <eref target="https://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
</t>
<t>
Working Group information can be found at <eref target="http://tools.ietf.org/wg/httpbis/"/>; that specific to HTTP/2
are at <eref target="http://http2.github.io/"/>.
</t>
<t>
The changes in this draft are summarized in <xref target="changes.since.draft-ietf-httpbis-header-compression-09"/>.
</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>
In HTTP/1.1 (see <xref target="RFC7230"/>), header fields are
not compressed. As Web pages have grown to require dozens to
hundreds of requests, the redundant header fields in these
requests unnecessarily consume bandwidth, measurably increasing
latency.
</t>
<t>
<xref target="SPDY">SPDY</xref> initially addressed this
redundancy by compressing header fields using the <xref target="DEFLATE">DEFLATE</xref> format, which proved very
effective at efficiently representing the redundant header
fields. However, that approach exposed a security risk as
demonstrated by the CRIME attack (see <xref target="CRIME"/>).
</t>
<t>
This specification defines HPACK, a new compressor for header
fields which eliminates redundant header fields, limits
vulnerability to known security attacks, and which has a bounded
memory requirement for use in constrained environments.
Potential security concerns for HPACK are described in <xref target="Security"/>.
</t>
<t>
The HPACK format is intentionally simple and inflexible. Both
characteristics reduce the risk of interoperability or security
issues due to implementation error. No extensibility
mechanisms are defined; changes to the format are only possible
by defining a complete replacement.
</t>
<section title="Overview">
<t>
The format defined in this specification treats a list of
header fields as an ordered collection of name-value pairs
that can include duplicate pairs. Names and values are
considered to be opaque sequences of octets, and the order
of header fields is preserved after being compressed and
decompressed.
</t>
<t>
Encoding is informed by header field tables that map
header fields to indexed values. These header field tables
can be incrementally updated as new header fields are
encoded or decoded.
</t>
<t>
In the encoded form, a header field is represented either
literally or as a reference to a header field in one of
the header field tables. Therefore, a list of header fields
can be encoded using a mixture of references and literal
values.
</t>
<t>
Literal values are either encoded directly or using a static
Huffman code.
</t>
<t>
The encoder is responsible for deciding which header fields
to insert as new entries in the header field tables. The
decoder executes the modifications to the header field
tables prescribed by the encoder, reconstructing the list of
header fields in the process. This enables decoders to
remain simple and interoperate with a wide variety of
encoders.
</t>
<t>
Examples illustrating the use of these different mechanisms
to represent header fields are available in <xref target="examples"/>.
</t>
</section>
<section anchor="conventions" title="Conventions">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in <xref target="RFC2119">RFC 2119</xref>.
</t>
<t>
All numeric values are in network byte order. Values are
unsigned unless otherwise indicated. Literal values are
provided in decimal or hexadecimal as appropriate.
</t>
</section>
<section anchor="encoding.concepts" title="Terminology">
<t>
This specification uses the following terms:
<list style="hanging">
<t hangText="Header Field:">
A name-value pair. Both the name and value are
treated as opaque sequences of octets.
</t>
<t hangText="Dynamic Table:">
The dynamic table (see <xref target="dynamic.table"/>) is a table that
associates stored header fields with index values.
This table is dynamic and specific to an encoding or
decoding context.
</t>
<t hangText="Static Table:">
The static table (see <xref target="static.table"/>)
is a table that statically associates header fields
that occur frequently
with index values. This table is ordered,
read-only, always accessible, and may be shared
amongst all encoding or decoding contexts.
</t>
<t hangText="Header List:">
A header list is an ordered collection of header
fields that are encoded jointly, and can contain
duplicate header fields. A complete list of
header fields contained in an HTTP/2 header block
is a header list.
</t>
<t hangText="Header Field Representation:">
A header field can be represented in encoded form
either as a literal or as an index (see <xref target="header.representation"/>).
</t>
<t hangText="Header Block:">
An ordered list of header field representations
which, when decoded, yields a complete header list.
</t>
</list>
</t>
</section>
</section>
<section anchor="header.encoding" title="Compression Process Overview">
<t>
This specification does not describe a specific algorithm for an
encoder. Instead, it defines precisely how a decoder is
expected to operate, allowing encoders to produce any encoding
that this definition permits.
</t>
<section anchor="header.list.ordering" title="Header List Ordering">
<t>
HPACK preserves the ordering of header fields inside the
header list. An encoder MUST order header field
representations in the header block according to their
ordering in the original header list. A decoder MUST
order header fields in the decoded header list according to
their ordering in the header block.
</t>
</section>
<section anchor="encoding.context" title="Encoding and Decoding Contexts">
<t>
To decompress header blocks, a decoder only needs to
maintain a dynamic table (see <xref target="dynamic.table"/>) as a decoding context. No
other dynamic state is needed.
</t>
<t>
When used for bidirectional communication, such as in HTTP,
the encoding and decoding dynamic tables maintained by an
endpoint are completely independent. I.e., the request
and response dynamic tables are separate.
</t>
</section>
<section anchor="indexing.tables" title="Indexing Tables">
<t>
HPACK uses two tables for associating header fields to
indexes. The static table (see <xref target="static.table"/>) is predefined and contains
common header fields (most of them with an empty value). The
dynamic table (see <xref target="dynamic.table"/>) is
dynamic and can be used by the encoder to index header
fields repeated in the encoded header lists.
</t>
<t>
These two tables are combined into a single address space
for defining index values (see <xref target="index.address.space"/>).
</t>
<section anchor="static.table" title="Static Table">
<t>
The static table consists of a predefined static list of
header fields. Its entries are defined in <xref target="static.table.definition"/>.
</t>
</section>
<section anchor="dynamic.table" title="Dynamic Table">
<t>
The dynamic table consists of a list of header fields
maintained in first-in, first-out order. The first and
newest entry in a dynamic table is at the lowest index,
and the oldest entry of a dynamic table is at the
highest index.
</t>
<t>
The dynamic table is initially empty. Entries are added
as each header block is decompressed.
</t>
<t>
The dynamic table can contain duplicate entries (i.e.,
entries with the same name and same value).
Therefore, duplicate entries MUST NOT be treated as an
error by a decoder.
</t>
<t>
The encoder decides how to update the dynamic table and
as such can control how much memory is used by the
dynamic table. To limit the memory requirements of the
decoder, the dynamic table size is strictly bounded (see
<xref target="maximum.table.size"/>).
</t>
<t>
The decoder updates the dynamic table during the
processing of a list of header field representations
(see <xref target="header.representation.processing"/>).
</t>
</section>
<section anchor="index.address.space" title="Index Address Space">
<t>
The static table and the dynamic table are combined into
a single index address space.
</t>
<t>
Indices between 1 and the length of the static table
(inclusive) refer to elements in the static table (see
<xref target="static.table"/>).
</t>
<t>
Indices strictly greater than the length of the static
table refer to elements in the dynamic table (see <xref target="dynamic.table"/>). The length
of the static table is subtracted to find the index into
the dynamic table.
</t>
<t>
Indices strictly greater than the sum of the lengths of
both tables MUST be treated as a decoding error.
</t>
<figure anchor="Index.Address.Space" title="Index Address Space">
<preamble>
For a static table size of s and a dynamic table
size of k, the following diagram shows the entire
valid index address space.
</preamble>
<artwork type="drawing"><![CDATA[
<---------- Index Address Space ---------->
<-- Static Table --> <-- Dynamic Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | s | |s+1| ... |s+k|
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Dropping Point
]]></artwork>
</figure>
</section>
</section>
<section anchor="header.representation" title="Header Field Representation">
<t>
An encoded header field can be represented either as an
index or as a literal.
</t>
<t>
An indexed representation defines a header field as a
reference to an entry in either the static table or the
dynamic table (see <xref target="indexed.header.representation"/>).
</t>
<t>
A literal representation defines a header field by
specifying its name and value. The header field name can be
represented literally or as a reference to an entry in
either the static table or the dynamic table. The header
field value is represented literally.
</t>
<t>
Three different literal representations are defined:
<list style="symbols">
<t>
A literal representation that adds the header field
as a new entry at the beginning of the dynamic table
(see <xref target="literal.header.with.incremental.indexing"/>).
</t>
<t>
A literal representation that does not add the
header field to the dynamic table (see <xref target="literal.header.without.indexing"/>).
</t>
<t>
A literal representation that does not add the
header field to the dynamic table, with the
additional stipulation that this header field always
use a literal representation, in particular when
re-encoded by an intermediary (see <xref target="literal.header.never.indexed"/>). This
representation is intended for protecting header
field values that are not to be put at risk by
compressing them (see <xref target="never.indexed.literals"/> for more
details).
</t>
</list>
</t>
<t>
The selection of one of these literal representations can be
guided by security considerations, in order to protect
sensitive header field values (see <xref target="compression.based.attacks"/>).
</t>
<t>
The literal representation of a header field name or of a
header field value can encode the sequence of octets either
directly or using a static Huffman code (see <xref target="string.literal.representation"/>).
</t>
</section>
</section>
<section anchor="header.block.decoding" title="Header Block Decoding">
<section anchor="header.block.processing" title="Header Block Processing">
<t>
A decoder processes a header block sequentially to
reconstruct the original header list.
</t>
<t>
A header block is the concatenation of header field
representations. The different possible header field
representations are described in <xref target="detailed.format"/>.
</t>
<t>
Once a header field is decoded and added to the
reconstructed header list, the header field cannot be
removed. A header field added to the header list can be
safely passed to the application.
</t>
<t>
By passing the resulting header fields to the application,
a decoder can be implemented with minimal transitory memory
commitment in addition to the dynamic table.
</t>
</section>
<section anchor="header.representation.processing" title="Header Field Representation Processing">
<t>
The processing of a header block to obtain a header list is
defined in this section. To ensure that the decoding will
successfully produce a header list, a decoder MUST obey the
following rules.
</t>
<t>
All the header field representations contained in a header
block are processed in the order in which they appear, as
specified below. Details on the formatting of the various
header field representations, and some additional processing
instructions are found in <xref target="detailed.format"/>.
</t>
<t>
An <spanx>indexed representation</spanx> entails the
following actions:
<list style="symbols">
<t>
The header field corresponding to the referenced
entry in either the static table or dynamic table is
appended to the decoded header list.
</t>
</list>
</t>
<t>
A <spanx>literal representation</spanx> that is <spanx>not
added</spanx> to the dynamic table entails the following
action:
<list style="symbols">
<t>
The header field is appended to the decoded header
list.
</t>
</list>
</t>
<t>
A <spanx>literal representation</spanx> that is
<spanx>added</spanx> to the dynamic table entails the
following actions:
<list style="symbols">
<t>
The header field is appended to the decoded header
list.
</t>
<t>
The header field is inserted at the beginning of the
dynamic table. This insertion could result in the
eviction of previous entries in the dynamic table
(see <xref target="entry.addition"/>).
</t>
</list>
</t>
</section>
</section>
<section anchor="dynamic.table.management" title="Dynamic Table Management">
<t>
To limit the memory requirements on the decoder side, the
dynamic table is constrained in size.
</t>
<section anchor="calculating.table.size" title="Calculating Table Size">
<t>
The size of the dynamic table is the sum of the size of its
entries.
</t>
<t>
The size of an entry is the sum of its name's length in
octets (as defined in <xref target="string.literal.representation"/>), its value's
length in octets, plus 32.
</t>
<t>
The size of an entry is calculated using the length of its
name and value without any Huffman encoding applied.
</t>
<t>
<list style="hanging">
<t hangText="Note:">
The additional 32 octets account for an estimated
overhead associated with an entry. For example, an
entry structure using two 64-bit pointers to
reference the name and the value of the entry, and
two 64-bit integers for counting the number of
references to the name and value would have 32
octets of overhead.
</t>
</list>
</t>
</section>
<section anchor="maximum.table.size" title="Maximum Table Size">
<t>
Protocols that use HPACK determine the maximum size that the
encoder is permitted to use for the dynamic table. In
HTTP/2, this value is determined by the
SETTINGS_HEADER_TABLE_SIZE setting (see Section 6.5.2 of <xref target="HTTP2"/>).
</t>
<t>
An encoder can choose to use less capacity than this maximum
size (see <xref target="encoding.context.update"/>), but the
chosen size MUST stay lower than or equal to the maximum set
by the protocol.
</t>
<t>
A change in the maximum size of the dynamic table is
signaled via an encoding context update (see <xref target="encoding.context.update"/>). This encoding context
update MUST occur at the beginning of the first header block
following the change to the dynamic table size. In HTTP/2,
this follows a settings acknowledgment (see Section 6.5.3 of <xref target="HTTP2"/>).
</t>
<t>
Multiple updates to the maximum table size can occur between
the transmission of two header blocks. In the case that this
size is changed more than once in this interval, the
smallest maximum table size that occurs in that interval
MUST be signaled in an encoding context update. The final
maximum size is always signaled, resulting in at most two
encoding context updates. This ensures that the decoder is
able to perform eviction based on reductions in dynamic
table size (see <xref target="entry.eviction"/>).
</t>
<t>
This mechanism can be used to completely clear entries from
the dynamic table by setting a maximum size of 0, which can
subsequently be restored.
</t>
</section>
<section anchor="entry.eviction" title="Entry Eviction when Dynamic Table Size Changes">
<t>
Whenever the maximum size for the dynamic table is reduced,
entries are evicted from the end of the dynamic table until
the size of the dynamic table is less than or equal to the
maximum size.
</t>
</section>
<section anchor="entry.addition" title="Entry Eviction when Adding New Entries">
<t>
Before a new entry is added to the dynamic table, entries
are evicted from the end of the dynamic table until the size
of the dynamic table is less than or equal to (maximum size
- new entry size), or until the table is empty.
</t>
<t>
If the size of the new entry is less than or equal to the
maximum size, that entry is added to the table. It is not
an error to attempt to add an entry that is larger than the
maximum size; an attempt to add an entry larger than the
maximum size causes the table to be emptied of all existing
entries, and results in an empty table.
</t>
<t>
A new entry can reference the name of an entry in the
dynamic table that will be evicted when adding this new
entry into the dynamic table. Implementations are cautioned
to avoid deleting the referenced name if the referenced
entry is evicted from the dynamic table prior to inserting
the new entry.
</t>
</section>
</section>
<section anchor="low-level.representation" title="Primitive Type Representations">
<t>
HPACK encoding uses two primitive types: unsigned variable
length integers, and strings of octets.
</t>
<section anchor="integer.representation" title="Integer Representation">
<t>
Integers are used to represent name indexes, header field
indexes or string lengths. An integer representation can
start anywhere within an octet. To allow for optimized
processing, an integer representation always finishes at the
end of an octet.
</t>
<t>
An integer is represented in two parts: a prefix that fills
the current octet and an optional list of octets that are
used if the integer value does not fit within the prefix.
The number of bits of the prefix (called N) is a parameter
of the integer representation.
</t>
<t>
If the integer value is small enough, i.e., strictly less
than 2^N-1, it is encoded within the N-bit
prefix.
</t>
<figure anchor="Integer.Value.Encoded.within.the.Prefix.shown.for.N.5" title="Integer Value Encoded within the Prefix (shown for N = 5)">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | Value |
+---+---+---+-------------------+
]]></artwork>
</figure>
<t>
Otherwise, all the bits of the prefix are set to 1 and the
value, decreased by 2^N-1, is encoded using a
list of one or more octets. The most significant bit of each
octet is used as a continuation flag: its value is set to 1
except for the last octet in the list. The remaining bits of
the octets are used to encode the decreased value.
</t>
<figure anchor="Integer.Value.Encoded.after.the.Prefix.shown.for.N.5" title="Integer Value Encoded after the Prefix (shown for N = 5)">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | 1 1 1 1 1 |
+---+---+---+-------------------+
| 1 | Value-(2^N-1) LSB |
+---+---------------------------+
...
+---+---------------------------+
| 0 | Value-(2^N-1) MSB |
+---+---------------------------+
]]></artwork>
</figure>
<t>
Decoding the integer value from the list of octets starts by
reversing the order of the octets in the list. Then, for
each octet, its most significant bit is removed. The
remaining bits of the octets are concatenated and the
resulting value is increased by 2^N-1 to
obtain the integer value.
</t>
<t>
The prefix size, N, is always between 1 and 8 bits. An
integer starting at an octet-boundary will have an 8-bit
prefix.
</t>
<figure>
<preamble>
Pseudo-code to represent an integer I is as follows:
</preamble>
<artwork type="inline"><![CDATA[
if I < 2^N - 1, encode I on N bits
else
encode (2^N - 1) on N bits
I = I - (2^N - 1)
while I >= 128
encode (I % 128 + 128) on 8 bits
I = I / 128
encode I on 8 bits
]]></artwork>
</figure>
<figure>
<preamble>
Pseudo-code to decode an integer I is as follows:
</preamble>
<artwork type="inline"><![CDATA[
decode I from the next N bits
if I < 2^N - 1, return I
else
M = 0
repeat
B = next octet
I = I + (B & 127) * 2^M
M = M + 7
while B & 128 == 128
return I
]]></artwork>
</figure>
<t>
Examples illustrating the encoding of integers are available
in <xref target="integer.representation.examples"/>.
</t>
<t>
This integer representation allows for values of indefinite
size. It is also possible for an encoder to send a large
number of zero values, which can waste octets and could be
used to overflow integer values. Integer encodings that
exceed an implementation limits - in value or octet length -
MUST be treated as a decoding error. Different limits can
be set for each of the different uses of integers, based on
implementation constraints.
</t>
</section>
<section anchor="string.literal.representation" title="String Literal Representation">
<t>
Header field names and header field values can be
represented as literal strings. A literal string is encoded
as a sequence of octets, either by directly encoding the
literal string's octets, or by using a Huffman code
(see <xref target="HUFFMAN"/>).
</t>
<figure anchor="String.Literal.Representation" title="String Literal Representation">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| H | String Length (7+) |
+---+---------------------------+
| String Data (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal string representation contains the following
fields:
<list style="hanging">
<t hangText="H:">
A one bit flag, H, indicating whether or not the
octets of the string are Huffman encoded.
</t>
<t hangText="String Length:">
The number of octets used to encode the string
literal, encoded as an integer with 7-bit prefix
(see <xref target="integer.representation"/>).
</t>
<t hangText="String Data:">
The encoded data of the string literal. If H is
'0', then the encoded data is the raw octets of
the string literal. If H is '1', then the
encoded data is the Huffman encoding of the
string literal.
</t>
</list>
</t>
<t>
String literals which use Huffman encoding are encoded with
the Huffman code defined in <xref target="huffman.code"/>
(see examples for requests in <xref target="request.examples.with.huffman.coding"/> and for
responses in <xref target="response.examples.with.huffman.coding"/>). The
encoded data is the bitwise concatenation of the codes
corresponding to each octet of the string literal.
</t>
<t>
As the Huffman encoded data doesn't always end at an octet
boundary, some padding is inserted after it, up to the next
octet boundary. To prevent this padding to be misinterpreted
as part of the string literal, the most significant bits of
the code corresponding to the EOS (end-of-string) symbol are
used.
</t>
<t>
Upon decoding, an incomplete code at the end of the
encoded data is to be considered as padding and discarded. A
padding strictly longer than 7 bits MUST be treated as a
decoding error. A padding not corresponding to the most
significant bits of the code for the EOS symbol MUST be
treated as a decoding error. A Huffman encoded string
literal containing the EOS symbol MUST be treated as a
decoding error.
</t>
</section>
</section>
<section anchor="detailed.format" title="Binary Format">
<t>
This section describes the detailed format of each of the
different header field representations, plus the encoding
context update instruction.
</t>
<section anchor="indexed.header.representation" title="Indexed Header Field Representation">
<t>
An indexed header field representation identifies an entry
in either the static table or the dynamic table (see <xref target="indexing.tables"/>).
</t>
<t>
An indexed header field representation causes a
header field to be added to the decoded header list, as
described in <xref target="header.representation.processing"/>.
</t>
<figure anchor="Indexed.Header.Field" title="Indexed Header Field">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
]]></artwork>
</figure>
<t>
An indexed header field starts with the '1' 1-bit pattern,
followed by the index of the matching header field,
represented as an integer with a 7-bit prefix (see <xref target="integer.representation"/>).
</t>
<t>
The index value of 0 is not used. It MUST be treated as a
decoding error if found in an indexed header field
representation.
</t>
</section>
<section anchor="literal.header.representation" title="Literal Header Field Representation">
<t>
A literal header field representation contains a literal
header field value. Header field names are either provided
as a literal or by reference to an existing table entry,
either from the static table or the dynamic table (see <xref target="indexing.tables"/>).
</t>
<t>
This specification defines three forms of literal header
field representations; with indexing, without indexing,
and never indexed.
</t>
<section anchor="literal.header.with.incremental.indexing" title="Literal Header Field with Incremental Indexing">
<t>
A literal header field with incremental indexing
representation results in appending a header field to
the decoded header list and inserting it as a new entry
into the dynamic table.
</t>
<figure anchor="Literal.Header.Field.with.Incremental.Indexing.Indexed.Name" title="Literal Header Field with Incremental Indexing - Indexed Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<figure anchor="Literal.Header.Field.with.Incremental.Indexing.New.Name" title="Literal Header Field with Incremental Indexing - New Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal header field with incremental indexing
representation starts with the '01' 2-bit pattern.
</t>
<t>
If the header field name matches the header field name
of an entry stored in the static table or the dynamic
table, the header field name can be represented using
the index of that entry. In this case, the index of the
entry is represented as an integer with a 6-bit prefix
(see <xref target="integer.representation"/>). This
value is always non-zero.
</t>
<t>
Otherwise, the header field name is represented as a
literal string (see <xref target="string.literal.representation"/>). A value
0 is used in place of the 6-bit index, followed by the
header field name.
</t>
<t>
Either form of header field name representation is
followed by the header field value represented as a
literal string (see <xref target="string.literal.representation"/>).
</t>
</section>
<section anchor="literal.header.without.indexing" title="Literal Header Field without Indexing">
<t>
A literal header field without indexing representation
results in appending a header field to the decoded
header list without altering the dynamic table.
</t>
<figure anchor="Literal.Header.Field.without.Indexing.Indexed.Name" title="Literal Header Field without Indexing - Indexed Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<figure anchor="Literal.Header.Field.without.Indexing.New.Name" title="Literal Header Field without Indexing - New Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal header field without indexing representation
starts with the '0000' 4-bit pattern.
</t>
<t>
If the header field name matches the header field name
of an entry stored in the static table or the dynamic
table, the header field name can be represented using
the index of that entry. In this case, the index of the
entry is represented as an integer with a 4-bit prefix
(see <xref target="integer.representation"/>). This
value is always non-zero.
</t>
<t>
Otherwise, the header field name is represented as a
literal string (see <xref target="string.literal.representation"/>). A value
0 is used in place of the 4-bit index, followed by the
header field name.
</t>
<t>
Either form of header field name representation is
followed by the header field value represented as a
literal string (see <xref target="string.literal.representation"/>).
</t>
</section>
<section anchor="literal.header.never.indexed" title="Literal Header Field never Indexed">
<t>
A literal header field never indexed representation
results in appending a header field to the decoded
header list without altering the dynamic table.
Intermediaries MUST use the same representation for
encoding this header field.
</t>
<figure anchor="Literal.Header.Field.never.Indexed.Indexed.Name" title="Literal Header Field never Indexed - Indexed Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<figure anchor="Literal.Header.Field.never.Indexed.New.Name" title="Literal Header Field never Indexed - New Name">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
]]></artwork>
</figure>
<t>
A literal header field never indexed representation
starts with the '0001' 4-bit pattern.
</t>
<t>
When a header field is represented as a literal header
field never indexed, it MUST always be encoded with
this specific literal representation. In particular,
when a peer sends a header field that it received
represented as a literal header field never indexed, it
MUST use the same representation to forward this header
field.
</t>
<t>
This representation is intended for protecting header
field values that are not to be put at risk by
compressing them (see <xref target="compression.based.attacks"/> for more details).
</t>
<t>
The encoding of the representation is identical to the
literal header field without indexing
(see <xref target="literal.header.without.indexing"/>).
</t>
</section>
</section>
<section anchor="encoding.context.update" title="Dynamic Table Size Update">
<t>
A dynamic table size update signals a change to the size of
the dynamic table.
</t>
<figure anchor="Maximum.Dynamic.Table.Size.Change" title="Maximum Dynamic Table Size Change">
<artwork type="inline"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Max size (5+) |
+---+---------------------------+
]]></artwork>
</figure>
<t>
A dynamic table size update starts with the '001' 3-bit
pattern, followed by the new maximum size, represented as an
integer with a 5-bit prefix (see <xref target="integer.representation"/>).
</t>
<t>
The new maximum size MUST be lower than or equal to the last
value of the maximum size of the dynamic table. A value that
exceeds this limit MUST be treated as a decoding error. In
HTTP/2, this limit is the last value of the
SETTINGS_HEADER_TABLE_SIZE parameter (see Section 6.5.2 of <xref target="HTTP2"/>)
received from the decoder and acknowledged by the encoder
(see Section 6.5.3 of <xref target="HTTP2"/>).
</t>
<t>
Reducing the maximum size of the dynamic table can cause
entries to be evicted (see <xref target="entry.eviction"/>).
</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>
This section describes potential areas of security concern
with HPACK:
<list style="symbols">
<t>
Use of compression as a length-based oracle for
verifying guesses about secrets that are compressed
into a shared compression context.
</t>
<t>
Denial of service resulting from exhausting processing
or memory capacity at a decoder.
</t>
</list>
</t>
<section anchor="compression.based.attacks" title="Probing Dynamic Table State">
<t>
HPACK reduces the length of header field encodings by
exploiting the redundancy inherent in protocols like HTTP.
The ultimate goal of this is to reduce the amount of data
that is required to send HTTP requests or responses.
</t>
<t>
The compression context used to encode header fields can be
probed by an attacker who can both define header fields to
be encoded and transmitted and observe the length of those
fields once they are encoded. When an attacker can do both,
they can adaptively modify requests in order to confirm
guesses about the dynamic table state. If a guess is
compressed into a shorter length, the attacker can observe
the encoded length and infer that the guess was correct.
</t>
<t>
This is possible even over the Transport Layer Security
Protocol (TLS, see <xref target="TLS12"/>), because while
TLS provides confidentiality protection for content, it only
provides a limited amount of protection for the length of
that content.
<list style="hanging">
<t hangText="Note:">
Padding schemes only provide limited protection
against an attacker with these capabilities,
potentially only forcing an increased number of
guesses to learn the length associated with a given
guess. Padding schemes also work directly against
compression by increasing the number of bits that
are transmitted.
</t>
</list>
</t>
<t>
Attacks like <xref target="CRIME">CRIME</xref> demonstrated
the existence of these general attacker capabilities. The
specific attack exploited the fact that <xref target="DEFLATE">DEFLATE</xref> removes redundancy based
on prefix matching. This permitted the attacker to confirm
guesses a character at a time, reducing an exponential-time
attack into a linear-time attack.
</t>
<section title="Applicability to HPACK and HTTP">
<t>
HPACK mitigates but does not completely prevent attacks
modeled on <xref target="CRIME">CRIME</xref> by forcing
a guess to match an entire header field value, rather
than individual characters. An attacker can only learn
whether a guess is correct or not, so is reduced to a
brute force guess for the header field values.
</t>
<t>
The viability of recovering specific header field values
therefore depends on the entropy of values. As a
result, values with high entropy are unlikely to be
recovered successfully. However, values with low
entropy remain vulnerable.
</t>
<t>
Attacks of this nature are possible any time that two
mutually distrustful entities control requests or
responses that are placed onto a single HTTP/2
connection. If the shared HPACK compressor permits one
entity to add entries to the dynamic table, and the
other to access those entries, then the state of the
table can be learned.
</t>
<t>
Having requests or responses from mutually distrustful
entities occurs when an intermediary either:
<list style="symbols">
<t>
sends requests from multiple clients on a single
connection toward an origin server, or
</t>
<t>
takes responses from multiple origin servers and
places them on a shared connection toward a
client.
</t>
</list>
Web browsers also need to assume that requests made on
the same connection by different <xref target="ORIGIN">web origins</xref> are made by mutually
distrustful entities.
</t>
</section>
<section title="Mitigation">
<t>
Users of HTTP that require confidentiality for header
fields can use values with entropy sufficient to make
guessing infeasible. However, this is impractical as a
general solution because it forces all users of HTTP to
take steps to mitigate attacks. It would impose new
constraints on how HTTP is used.
</t>
<t>
Rather than impose constraints on users of HTTP, an
implementation of HPACK can instead constrain how
compression is applied in order to limit the potential
for dynamic table probing.
</t>
<t>
An ideal solution segregates access to the dynamic table
based on the entity that is constructing header fields.
Header field values that are added to the table are
attributed to an entity, and only the entity that
created a particular value can extract that value.
</t>
<t>
To improve compression performance of this option,
certain entries might be tagged as being public. For
example, a web browser might make the values of the
Accept-Encoding header field available in all requests.
</t>
<t>
An encoder without good knowledge of the provenance of
header fields might instead introduce a penalty for
a header field with many different values, such that a
large number of attempts to guess a header field
value results in the header field no more being compared
to the dynamic table entries in future messages,
effectively preventing further guesses.
<list style="hanging">
<t hangText="Note:">
Simply removing entries corresponding to the
header field from the dynamic table
can be ineffectual if the attacker has a
reliable way of causing values to be
reinstalled. For example, a request to load an
image in a web browser typically includes the
Cookie header field (a potentially highly valued
target for this sort of attack), and web sites
can easily force an image to be loaded, thereby
refreshing the entry in the dynamic table.
</t>
</list>
</t>
<t>
This response might be made inversely proportional to
the length of the header field value. Marking a header
field as not using the dynamic table any more
might occur for shorter values more quickly
or with higher probability than for longer values.
</t>
</section>
<section anchor="never.indexed.literals" title="Never Indexed Literals">
<t>
Implementations can also choose to protect sensitive
header fields by not compressing them and instead
encoding their value as literals.
</t>
<t>
Refusing to generate an indexed representation for a
header field is only effective if compression is avoided
on all hops. The never indexed literal (see <xref target="literal.header.never.indexed"/>) can be used
to signal to intermediaries that a particular value was
intentionally sent as a literal.
</t>
<t>
An intermediary MUST NOT re-encode a value that uses the
never indexed literal representation with another
representation that would index it. If HPACK is used
for re-encoding, the never indexed literal
representation MUST be used.
</t>
<t>
The choice to use a never indexed literal representation
for a header field depends on several factors. Since
HPACK doesn't protect against guessing an entire header
field value, short or low-entropy values are more
readily recovered by an adversary. Therefore, an encoder
might choose not to index values with low entropy.
</t>
<t>
An encoder might also choose not to index values for
header fields that are considered to be highly valuable
or sensitive to recovery, such as the Cookie or
Authorization header fields.
</t>
<t>
On the contrary, an encoder might prefer indexing values
for header fields that have little or no value if they
were exposed. For instance, a User-Agent header field
does not commonly vary between requests and is sent to
any server. In that case, confirmation that a particular
User-Agent value has been used provides little value.
</t>
<t>
Note that these criteria for deciding to use a never
indexed literal representation will evolve over time as
new attacks are discovered.
</t>
</section>
</section>
<section title="Static Huffman Encoding">
<t>
There is no currently known attack against a static Huffman
encoding. A study has shown that using a static Huffman
encoding table created an information leakage, however this
same study concluded that an attacker could not take
advantage of this information leakage to recover any
meaningful amount of information (see <xref target="PETAL"/>).
</t>
</section>
<section title="Memory Consumption">
<t>
An attacker can try to cause an endpoint to exhaust its
memory. HPACK is designed to limit both the peak and state
amounts of memory allocated by an endpoint.
</t>
<t>
The amount of memory used by the compressor is limited by
the protocol using HPACK through the definition of the
maximum size of the dynamic table.
In HTTP/2, this value is controlled by the decoder through
the setting parameter SETTINGS_HEADER_TABLE_SIZE (see Section 6.5.2 of <xref target="HTTP2"/>).
This limit takes into account both the size of the data
stored in the dynamic table, plus a small allowance for
overhead.
</t>
<t>
A decoder can limit the amount of state memory used by
setting an appropriate value for the maximum size of the
dynamic table. In HTTP/2, this is realized by setting an
appropriate value for the SETTINGS_HEADER_TABLE_SIZE
parameter. An encoder can limit the amount of state memory
it uses by signaling lower dynamic table size than the
decoder allows (see <xref target="encoding.context.update"/>).
</t>
<t>
The amount of temporary memory consumed by an encoder or
decoder can be limited by processing header fields
sequentially. An implementation does not need to retain a
complete list of header fields. Note however that it might
be necessary for an application to retain a complete header
list for other reasons; even though HPACK does not force
this to occur, application constraints might make this
necessary.
</t>
</section>
<section title="Implementation Limits">
<t>
An implementation of HPACK needs to ensure that large values
for integers, long encoding for integers, or long string
literals do not create security weaknesses.
</t>
<t>
An implementation has to set a limit for the values it
accepts for integers, as well as for the encoded length (see
<xref target="integer.representation"/>). In the same way,
it has to set a limit to the length it accepts for string
literals (see <xref target="string.literal.representation"/>).
</t>
</section>
</section>
<section title="IANA Considerations">
<t>
This document has no IANA actions.
</t>
</section>
<section title="Acknowledgments">
<t>
This specification includes substantial input from the following
individuals:
<list style="symbols">
<t>
Mike Bishop, Jeff Pinner, Julian Reschke, Martin Thomson
(substantial editorial contributions).
</t>
<t>
Johnny Graettinger (Huffman code statistics).
</t>
</list>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="HTTP2">
<front>
<title>Hypertext Transfer Protocol version 2</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe">
<organization>Twist</organization>
</author>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google</organization>
</author>
<author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor">
<organization>Mozilla</organization>
</author>
<date month="February" year="2015"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-http2-17"/>
</reference>
<reference anchor="RFC7230">
<front>
<title>
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and
Routing
</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="June" year="2014"/>
</front>
<seriesInfo name="RFC" value="7230"/>
</reference>
<reference anchor="RFC2119">
<front>
<title>
Key words for use in RFCs to Indicate Requirement Levels
</title>
<author initials="S." surname="Bradner" fullname="Scott Bradner">
<organization>Harvard University</organization>
<address><email>sob@harvard.edu</email></address>
</author>
<date month="March" year="1997"/>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="SPDY">
<front>
<title>SPDY Protocol</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe">
<organization>Twist</organization>
</author>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google</organization>
</author>
<date month="February" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-mbelshe-httpbis-spdy-00"/>
</reference>
<reference anchor="TLS12">
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
<author initials="T." surname="Dierks" fullname="Tim Dierks"/>
<author initials="E." surname="Rescorla" fullname="Eric Rescorla"/>
<date year="2008" month="August"/>
</front>
<seriesInfo name="RFC" value="5246"/>
</reference>
<reference anchor="ORIGIN">
<front>
<title>The Web Origin Concept</title>
<author initials="A." surname="Barth" fullname="Adam Barth"/>
<date month="December" year="2011"/>
</front>
<seriesInfo name="RFC" value="6454"/>
</reference>
<reference anchor="DEFLATE">
<front>
<title>DEFLATE Compressed Data Format Specification version 1.3</title>
<author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
<organization>Aladdin Enterprises</organization>
</author>
<date month="May" year="1996"/>
</front>
<seriesInfo name="RFC" value="1951"/>
</reference>
<reference anchor="CRIME" target="https://docs.google.com/a/twist.com/presentation/d/11eBmGiHbYcHR9gL5nDyZChu_-lCa2GizeuOfaLU2HOU">
<front>
<title>The CRIME Attack</title>
<author initials="J." surname="Rizzo" fullname="Juliano Rizzo"/>
<author initials="T." surname="Duong" fullname="Thai Duong"/>
<date month="September" year="2012"/>
</front>
</reference>
<reference anchor="HUFFMAN" target="http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4051119">
<front>
<title>A Method for the Construction of Minimum Redundancy
Codes</title>
<author surname="Huffman" initials="D. A." fullname="David A. Huffman"/>
<date month="September" year="1952"/>
</front>
<seriesInfo name="Proceedings of the Institute of Radio Engineers" value="Volume 40, Number 9, pp. 1098-1101"/>
</reference>
<reference anchor="CANONICAL" target="https://dl.acm.org/citation.cfm?id=363991">
<front>
<title>Generating a canonical prefix encoding</title>
<author surname="Schwartz" initials="E. S." fullname="Eugene. S. Schwartz"/>
<author surname="Kallick" initials="B." fullname="Bruce Kallick"/>
<date month="March" year="1964"/>
</front>
<seriesInfo name="Communications of the ACM" value="Volume 7 Issue 3, pp. 166-169"/>
</reference>
<reference anchor="PETAL" target="http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-13-106.pdf">
<front>
<title>PETAL: Preset Encoding Table Information
Leakage</title>
<author surname="Tan" initials="J." fullname="Jiaqi Tan"/>
<author surname="Nahata" initials="J." fullname="Jayvardhan Nahata"/>
<date month="April" year="2013"/>
</front>
</reference>
</references>
<section anchor="static.table.definition" title="Static Table Definition">
<t>
The static table (see <xref target="static.table"/>) consists in
a predefined and unchangeable list of header fields.
</t>
<t>
The static table was created by listing the most common
header fields that are valid for messages exchanged inside a
HTTP/2 connection. For header fields with a few frequent
values, an entry was added for each of these frequent values.
For other header fields, an entry was added with an empty
value.
</t>
<t>
The following table lists the predefined header fields that
make-up the static table.
</t>
<texttable title="Static Table Entries" anchor="static.table.entries">
<!-- An easy way to renumber these in vim, with mark a and b
delimiting the table entries:
:let @a=1 | 'a,'bs/>[0-9 ][0-9 ]*</\='>'.(@a+setreg('a',@a+1)).'<'/
-->
<ttcol>Index</ttcol>
<ttcol>Header Name</ttcol>
<ttcol>Header Value</ttcol>
<c>1</c><c>:authority</c><c/>
<c>2</c><c>:method</c><c>GET</c>
<c>3</c><c>:method</c><c>POST</c>
<c>4</c><c>:path</c><c>/</c>
<c>5</c><c>:path</c><c>/index.html</c>
<c>6</c><c>:scheme</c><c>http</c>
<c>7</c><c>:scheme</c><c>https</c>
<c>8</c><c>:status</c><c>200</c>
<c>9</c><c>:status</c><c>204</c>
<c>10</c><c>:status</c><c>206</c>
<c>11</c><c>:status</c><c>304</c>
<c>12</c><c>:status</c><c>400</c>
<c>13</c><c>:status</c><c>404</c>
<c>14</c><c>:status</c><c>500</c>
<c>15</c><c>accept-charset</c><c/>
<c>16</c><c>accept-encoding</c><c>gzip, deflate</c>
<c>17</c><c>accept-language</c><c/>
<c>18</c><c>accept-ranges</c><c/>
<c>19</c><c>accept</c><c/>
<c>20</c><c>access-control-allow-origin</c><c/>
<c>21</c><c>age</c><c/>
<c>22</c><c>allow</c><c/>
<c>23</c><c>authorization</c><c/>
<c>24</c><c>cache-control</c><c/>
<c>25</c><c>content-disposition</c><c/>
<c>26</c><c>content-encoding</c><c/>
<c>27</c><c>content-language</c><c/>
<c>28</c><c>content-length</c><c/>
<c>29</c><c>content-location</c><c/>
<c>30</c><c>content-range</c><c/>
<c>31</c><c>content-type</c><c/>
<c>32</c><c>cookie</c><c/>
<c>33</c><c>date</c><c/>
<c>34</c><c>etag</c><c/>
<c>35</c><c>expect</c><c/>
<c>36</c><c>expires</c><c/>
<c>37</c><c>from</c><c/>
<c>38</c><c>host</c><c/>
<c>39</c><c>if-match</c><c/>
<c>40</c><c>if-modified-since</c><c/>
<c>41</c><c>if-none-match</c><c/>
<c>42</c><c>if-range</c><c/>
<c>43</c><c>if-unmodified-since</c><c/>
<c>44</c><c>last-modified</c><c/>
<c>45</c><c>link</c><c/>
<c>46</c><c>location</c><c/>
<c>47</c><c>max-forwards</c><c/>
<c>48</c><c>proxy-authenticate</c><c/>
<c>49</c><c>proxy-authorization</c><c/>
<c>50</c><c>range</c><c/>
<c>51</c><c>referer</c><c/>
<c>52</c><c>refresh</c><c/>
<c>53</c><c>retry-after</c><c/>
<c>54</c><c>server</c><c/>
<c>55</c><c>set-cookie</c><c/>
<c>56</c><c>strict-transport-security</c><c/>
<c>57</c><c>transfer-encoding</c><c/>
<c>58</c><c>user-agent</c><c/>
<c>59</c><c>vary</c><c/>
<c>60</c><c>via</c><c/>
<c>61</c><c>www-authenticate</c><c/>
</texttable>
<t>
<xref target="static.table.entries"/> gives the index of each
entry in the static table.
</t>
</section>
<section anchor="huffman.code" title="Huffman Code">
<t>
The following Huffman code is used when encoding string literals
with a Huffman coding (see <xref target="string.literal.representation"/>).
</t>
<t>
This Huffman code was generated from statistics obtained on a
large sample of HTTP headers. It is a canonical Huffman code
(see <xref target="CANONICAL"/>) with some tweaking to ensure
that no symbol has a unique code length.
</t>
<t>
Each row in the table defines the code used to represent a
symbol:
<list style="hanging">
<t hangText="sym:">
The symbol to be represented. It is the decimal value of
an octet, possibly prepended with its ASCII
representation. A specific symbol, "EOS", is used to
indicate the end of a string literal.
</t>
<t hangText="code as bits:">
The Huffman code for the symbol represented as a base-2
integer, aligned on the most significant bit (MSB).
</t>
<t hangText="code as hex:">
The Huffman code for the symbol, represented as a
hexadecimal integer, aligned on the least significant
bit (LSB).
</t>
<t hangText="len:">
The number of bits for the code representing the symbol.
</t>
</list>
</t>
<t>
As an example, the code for the symbol 47 (corresponding to the
ASCII character "/") consists in the 6 bits "0", "1", "1", "0",
"0", "0". This corresponds to the value 0x18 (in hexadecimal)
encoded in 6 bits.
</t>
<figure>
<artwork><![CDATA[
code
code as bits as hex len
sym aligned to MSB aligned in
to LSB bits
( 0) |11111111|11000 1ff8 [13]
( 1) |11111111|11111111|1011000 7fffd8 [23]
( 2) |11111111|11111111|11111110|0010 fffffe2 [28]
( 3) |11111111|11111111|11111110|0011 fffffe3 [28]
( 4) |11111111|11111111|11111110|0100 fffffe4 [28]
( 5) |11111111|11111111|11111110|0101 fffffe5 [28]
( 6) |11111111|11111111|11111110|0110 fffffe6 [28]
( 7) |11111111|11111111|11111110|0111 fffffe7 [28]
( 8) |11111111|11111111|11111110|1000 fffffe8 [28]
( 9) |11111111|11111111|11101010 ffffea [24]
( 10) |11111111|11111111|11111111|111100 3ffffffc [30]
( 11) |11111111|11111111|11111110|1001 fffffe9 [28]
( 12) |11111111|11111111|11111110|1010 fffffea [28]
( 13) |11111111|11111111|11111111|111101 3ffffffd [30]
( 14) |11111111|11111111|11111110|1011 fffffeb [28]
( 15) |11111111|11111111|11111110|1100 fffffec [28]
( 16) |11111111|11111111|11111110|1101 fffffed [28]
( 17) |11111111|11111111|11111110|1110 fffffee [28]
( 18) |11111111|11111111|11111110|1111 fffffef [28]
( 19) |11111111|11111111|11111111|0000 ffffff0 [28]
( 20) |11111111|11111111|11111111|0001 ffffff1 [28]
( 21) |11111111|11111111|11111111|0010 ffffff2 [28]
( 22) |11111111|11111111|11111111|111110 3ffffffe [30]
( 23) |11111111|11111111|11111111|0011 ffffff3 [28]
( 24) |11111111|11111111|11111111|0100 ffffff4 [28]
( 25) |11111111|11111111|11111111|0101 ffffff5 [28]
( 26) |11111111|11111111|11111111|0110 ffffff6 [28]
( 27) |11111111|11111111|11111111|0111 ffffff7 [28]
( 28) |11111111|11111111|11111111|1000 ffffff8 [28]
( 29) |11111111|11111111|11111111|1001 ffffff9 [28]
( 30) |11111111|11111111|11111111|1010 ffffffa [28]
( 31) |11111111|11111111|11111111|1011 ffffffb [28]
' ' ( 32) |010100 14 [ 6]
'!' ( 33) |11111110|00 3f8 [10]
'"' ( 34) |11111110|01 3f9 [10]
'#' ( 35) |11111111|1010 ffa [12]
'$' ( 36) |11111111|11001 1ff9 [13]
'%' ( 37) |010101 15 [ 6]
'&' ( 38) |11111000 f8 [ 8]
''' ( 39) |11111111|010 7fa [11]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111110|11 3fb [10]
'*' ( 42) |11111001 f9 [ 8]
'+' ( 43) |11111111|011 7fb [11]
',' ( 44) |11111010 fa [ 8]
'-' ( 45) |010110 16 [ 6]
'.' ( 46) |010111 17 [ 6]
'/' ( 47) |011000 18 [ 6]
'0' ( 48) |00000 0 [ 5]
'1' ( 49) |00001 1 [ 5]
'2' ( 50) |00010 2 [ 5]
'3' ( 51) |011001 19 [ 6]
'4' ( 52) |011010 1a [ 6]
'5' ( 53) |011011 1b [ 6]
'6' ( 54) |011100 1c [ 6]
'7' ( 55) |011101 1d [ 6]
'8' ( 56) |011110 1e [ 6]
'9' ( 57) |011111 1f [ 6]
':' ( 58) |1011100 5c [ 7]
';' ( 59) |11111011 fb [ 8]
'<' ( 60) |11111111|1111100 7ffc [15]
'=' ( 61) |100000 20 [ 6]
'>' ( 62) |11111111|1011 ffb [12]
'?' ( 63) |11111111|00 3fc [10]
'@' ( 64) |11111111|11010 1ffa [13]
'A' ( 65) |100001 21 [ 6]
'B' ( 66) |1011101 5d [ 7]
'C' ( 67) |1011110 5e [ 7]
'D' ( 68) |1011111 5f [ 7]
'E' ( 69) |1100000 60 [ 7]
'F' ( 70) |1100001 61 [ 7]
'G' ( 71) |1100010 62 [ 7]
'H' ( 72) |1100011 63 [ 7]
'I' ( 73) |1100100 64 [ 7]
'J' ( 74) |1100101 65 [ 7]
'K' ( 75) |1100110 66 [ 7]
'L' ( 76) |1100111 67 [ 7]
'M' ( 77) |1101000 68 [ 7]
'N' ( 78) |1101001 69 [ 7]
'O' ( 79) |1101010 6a [ 7]
'P' ( 80) |1101011 6b [ 7]
'Q' ( 81) |1101100 6c [ 7]
'R' ( 82) |1101101 6d [ 7]
'S' ( 83) |1101110 6e [ 7]
'T' ( 84) |1101111 6f [ 7]
'U' ( 85) |1110000 70 [ 7]
'V' ( 86) |1110001 71 [ 7]
'W' ( 87) |1110010 72 [ 7]
'X' ( 88) |11111100 fc [ 8]
'Y' ( 89) |1110011 73 [ 7]
'Z' ( 90) |11111101 fd [ 8]
'[' ( 91) |11111111|11011 1ffb [13]
'\' ( 92) |11111111|11111110|000 7fff0 [19]
']' ( 93) |11111111|11100 1ffc [13]
'^' ( 94) |11111111|111100 3ffc [14]
'_' ( 95) |100010 22 [ 6]
'`' ( 96) |11111111|1111101 7ffd [15]
'a' ( 97) |00011 3 [ 5]
'b' ( 98) |100011 23 [ 6]
'c' ( 99) |00100 4 [ 5]
'd' (100) |100100 24 [ 6]
'e' (101) |00101 5 [ 5]
'f' (102) |100101 25 [ 6]
'g' (103) |100110 26 [ 6]
'h' (104) |100111 27 [ 6]
'i' (105) |00110 6 [ 5]
'j' (106) |1110100 74 [ 7]
'k' (107) |1110101 75 [ 7]
'l' (108) |101000 28 [ 6]
'm' (109) |101001 29 [ 6]
'n' (110) |101010 2a [ 6]
'o' (111) |00111 7 [ 5]
'p' (112) |101011 2b [ 6]
'q' (113) |1110110 76 [ 7]
'r' (114) |101100 2c [ 6]
's' (115) |01000 8 [ 5]
't' (116) |01001 9 [ 5]
'u' (117) |101101 2d [ 6]
'v' (118) |1110111 77 [ 7]
'w' (119) |1111000 78 [ 7]
'x' (120) |1111001 79 [ 7]
'y' (121) |1111010 7a [ 7]
'z' (122) |1111011 7b [ 7]
'{' (123) |11111111|1111110 7ffe [15]
'|' (124) |11111111|100 7fc [11]
'}' (125) |11111111|111101 3ffd [14]
'~' (126) |11111111|11101 1ffd [13]
(127) |11111111|11111111|11111111|1100 ffffffc [28]
(128) |11111111|11111110|0110 fffe6 [20]
(129) |11111111|11111111|010010 3fffd2 [22]
(130) |11111111|11111110|0111 fffe7 [20]
(131) |11111111|11111110|1000 fffe8 [20]
(132) |11111111|11111111|010011 3fffd3 [22]
(133) |11111111|11111111|010100 3fffd4 [22]
(134) |11111111|11111111|010101 3fffd5 [22]
(135) |11111111|11111111|1011001 7fffd9 [23]
(136) |11111111|11111111|010110 3fffd6 [22]
(137) |11111111|11111111|1011010 7fffda [23]
(138) |11111111|11111111|1011011 7fffdb [23]
(139) |11111111|11111111|1011100 7fffdc [23]
(140) |11111111|11111111|1011101 7fffdd [23]
(141) |11111111|11111111|1011110 7fffde [23]
(142) |11111111|11111111|11101011 ffffeb [24]
(143) |11111111|11111111|1011111 7fffdf [23]
(144) |11111111|11111111|11101100 ffffec [24]
(145) |11111111|11111111|11101101 ffffed [24]
(146) |11111111|11111111|010111 3fffd7 [22]
(147) |11111111|11111111|1100000 7fffe0 [23]
(148) |11111111|11111111|11101110 ffffee [24]
(149) |11111111|11111111|1100001 7fffe1 [23]
(150) |11111111|11111111|1100010 7fffe2 [23]
(151) |11111111|11111111|1100011 7fffe3 [23]
(152) |11111111|11111111|1100100 7fffe4 [23]
(153) |11111111|11111110|11100 1fffdc [21]
(154) |11111111|11111111|011000 3fffd8 [22]
(155) |11111111|11111111|1100101 7fffe5 [23]
(156) |11111111|11111111|011001 3fffd9 [22]
(157) |11111111|11111111|1100110 7fffe6 [23]
(158) |11111111|11111111|1100111 7fffe7 [23]
(159) |11111111|11111111|11101111 ffffef [24]
(160) |11111111|11111111|011010 3fffda [22]
(161) |11111111|11111110|11101 1fffdd [21]
(162) |11111111|11111110|1001 fffe9 [20]
(163) |11111111|11111111|011011 3fffdb [22]
(164) |11111111|11111111|011100 3fffdc [22]
(165) |11111111|11111111|1101000 7fffe8 [23]
(166) |11111111|11111111|1101001 7fffe9 [23]
(167) |11111111|11111110|11110 1fffde [21]
(168) |11111111|11111111|1101010 7fffea [23]
(169) |11111111|11111111|011101 3fffdd [22]
(170) |11111111|11111111|011110 3fffde [22]
(171) |11111111|11111111|11110000 fffff0 [24]
(172) |11111111|11111110|11111 1fffdf [21]
(173) |11111111|11111111|011111 3fffdf [22]
(174) |11111111|11111111|1101011 7fffeb [23]
(175) |11111111|11111111|1101100 7fffec [23]
(176) |11111111|11111111|00000 1fffe0 [21]
(177) |11111111|11111111|00001 1fffe1 [21]
(178) |11111111|11111111|100000 3fffe0 [22]
(179) |11111111|11111111|00010 1fffe2 [21]
(180) |11111111|11111111|1101101 7fffed [23]
(181) |11111111|11111111|100001 3fffe1 [22]
(182) |11111111|11111111|1101110 7fffee [23]
(183) |11111111|11111111|1101111 7fffef [23]
(184) |11111111|11111110|1010 fffea [20]
(185) |11111111|11111111|100010 3fffe2 [22]
(186) |11111111|11111111|100011 3fffe3 [22]
(187) |11111111|11111111|100100 3fffe4 [22]
(188) |11111111|11111111|1110000 7ffff0 [23]
(189) |11111111|11111111|100101 3fffe5 [22]
(190) |11111111|11111111|100110 3fffe6 [22]
(191) |11111111|11111111|1110001 7ffff1 [23]
(192) |11111111|11111111|11111000|00 3ffffe0 [26]
(193) |11111111|11111111|11111000|01 3ffffe1 [26]
(194) |11111111|11111110|1011 fffeb [20]
(195) |11111111|11111110|001 7fff1 [19]
(196) |11111111|11111111|100111 3fffe7 [22]
(197) |11111111|11111111|1110010 7ffff2 [23]
(198) |11111111|11111111|101000 3fffe8 [22]
(199) |11111111|11111111|11110110|0 1ffffec [25]
(200) |11111111|11111111|11111000|10 3ffffe2 [26]
(201) |11111111|11111111|11111000|11 3ffffe3 [26]
(202) |11111111|11111111|11111001|00 3ffffe4 [26]
(203) |11111111|11111111|11111011|110 7ffffde [27]
(204) |11111111|11111111|11111011|111 7ffffdf [27]
(205) |11111111|11111111|11111001|01 3ffffe5 [26]
(206) |11111111|11111111|11110001 fffff1 [24]
(207) |11111111|11111111|11110110|1 1ffffed [25]
(208) |11111111|11111110|010 7fff2 [19]
(209) |11111111|11111111|00011 1fffe3 [21]
(210) |11111111|11111111|11111001|10 3ffffe6 [26]
(211) |11111111|11111111|11111100|000 7ffffe0 [27]
(212) |11111111|11111111|11111100|001 7ffffe1 [27]
(213) |11111111|11111111|11111001|11 3ffffe7 [26]
(214) |11111111|11111111|11111100|010 7ffffe2 [27]
(215) |11111111|11111111|11110010 fffff2 [24]
(216) |11111111|11111111|00100 1fffe4 [21]
(217) |11111111|11111111|00101 1fffe5 [21]
(218) |11111111|11111111|11111010|00 3ffffe8 [26]
(219) |11111111|11111111|11111010|01 3ffffe9 [26]
(220) |11111111|11111111|11111111|1101 ffffffd [28]
(221) |11111111|11111111|11111100|011 7ffffe3 [27]
(222) |11111111|11111111|11111100|100 7ffffe4 [27]
(223) |11111111|11111111|11111100|101 7ffffe5 [27]
(224) |11111111|11111110|1100 fffec [20]
(225) |11111111|11111111|11110011 fffff3 [24]
(226) |11111111|11111110|1101 fffed [20]
(227) |11111111|11111111|00110 1fffe6 [21]
(228) |11111111|11111111|101001 3fffe9 [22]
(229) |11111111|11111111|00111 1fffe7 [21]
(230) |11111111|11111111|01000 1fffe8 [21]
(231) |11111111|11111111|1110011 7ffff3 [23]
(232) |11111111|11111111|101010 3fffea [22]
(233) |11111111|11111111|101011 3fffeb [22]
(234) |11111111|11111111|11110111|0 1ffffee [25]
(235) |11111111|11111111|11110111|1 1ffffef [25]
(236) |11111111|11111111|11110100 fffff4 [24]
(237) |11111111|11111111|11110101 fffff5 [24]
(238) |11111111|11111111|11111010|10 3ffffea [26]
(239) |11111111|11111111|1110100 7ffff4 [23]
(240) |11111111|11111111|11111010|11 3ffffeb [26]
(241) |11111111|11111111|11111100|110 7ffffe6 [27]
(242) |11111111|11111111|11111011|00 3ffffec [26]
(243) |11111111|11111111|11111011|01 3ffffed [26]
(244) |11111111|11111111|11111100|111 7ffffe7 [27]
(245) |11111111|11111111|11111101|000 7ffffe8 [27]
(246) |11111111|11111111|11111101|001 7ffffe9 [27]
(247) |11111111|11111111|11111101|010 7ffffea [27]
(248) |11111111|11111111|11111101|011 7ffffeb [27]
(249) |11111111|11111111|11111111|1110 ffffffe [28]
(250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27]
(252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27]
(254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
]]></artwork>
</figure>
</section>
<section anchor="examples" title="Examples">
<t>
A number of examples are worked through here, covering integer
encoding, header field representation, and the encoding of whole
lists of header fields, for both requests and responses, and
with and without Huffman coding.
</t>
<section anchor="integer.representation.examples" title="Integer Representation Examples">
<t>
This section shows the representation of integer values in
details (see <xref target="integer.representation"/>).
</t>
<section anchor="integer.representation.example1" title="Example 1: Encoding 10 Using a 5-bit Prefix">
<t>
The value 10 is to be encoded with a 5-bit prefix.
<list style="symbols">
<t>
10 is less than 31 (2^5 - 1) and
is represented using the 5-bit prefix.
</t>
</list>
</t>
<figure>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits
+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
</section>
<section anchor="integer.representation.example2" title="Example 2: Encoding 1337 Using a 5-bit Prefix">
<t>
The value I=1337 is to be encoded with a 5-bit prefix.
<list>
<t>
1337 is greater than 31 (2^5 - 1).
</t>
<t>
<list>
<t>The 5-bit prefix is filled with its max
value (31).</t>
</list>
</t>
<t>I = 1337 - (2^5 - 1) = 1306.</t>
<t>
<list>
<t>I (1306) is greater than or equal to 128,
the while loop body executes:</t>
<t>
<list>
<t>I % 128 == 26</t>
<t>26 + 128 == 154</t>
<t>154 is encoded in 8 bits as:
10011010</t>
<t>I is set to 10 (1306 / 128 ==
10)</t>
<t>I is no longer greater than or
equal to 128, the while loop
terminates.</t>
</list>
</t>
<t>
I, now 10, is encoded in 8 bits as:
00001010.
</t>
</list>
</t>
<t>The process ends.</t>
</list>
</t>
<figure>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done
+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
</section>
<section anchor="integer.representation.example3" title="Example 3: Encoding 42 Starting at an Octet Boundary">
<t>
The value 42 is to be encoded starting at an
octet-boundary. This implies that a 8-bit prefix is
used.
<list style="symbols">
<t>
42 is less than 255 (2^8 - 1) and
is represented using the 8-bit prefix.
</t>
</list>
</t>
<figure>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits
+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
</section>
</section>
<!-- example-start -->
<section anchor="header.field.representation.examples" title="Header Field Representation Examples">
<t>
This section shows several independent representation examples.
</t>
<section title="Literal Header Field with Indexing">
<t>
The header field representation uses a literal name and a literal
value. The header field is added to the dynamic table.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
custom-key: custom-header]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus
746f 6d2d 6865 6164 6572 | tom-header]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0d | Literal value (len = 13)
6375 7374 6f6d 2d68 6561 6465 72 | custom-header
| -> custom-key: custom-head\
| er]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 55) custom-key: custom-header
Table size: 55]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
custom-key: custom-header]]></artwork>
</figure>
</t>
</section>
<section title="Literal Header Field without Indexing">
<t>
The header field representation uses an indexed name and a literal
value. The header field is not added to the dynamic table.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:path: /sample/path]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
040c 2f73 616d 706c 652f 7061 7468 | ../sample/path]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
04 | == Literal not indexed ==
| Indexed name (idx = 4)
| :path
0c | Literal value (len = 12)
2f73 616d 706c 652f 7061 7468 | /sample/path
| -> :path: /sample/path]]></artwork>
</figure>
</t>
<t>
Dynamic table (after decoding): empty.
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:path: /sample/path]]></artwork>
</figure>
</t>
</section>
<section title="Literal Header Field never Indexed">
<t>
The header field representation uses a literal name and a literal
value. The header field is not added to the dynamic table, and must
use the same representation if re-encoded by an intermediary.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
password: secret]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre
74 | t]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
10 | == Literal never indexed ==
08 | Literal name (len = 8)
7061 7373 776f 7264 | password
06 | Literal value (len = 6)
7365 6372 6574 | secret
| -> password: secret]]></artwork>
</figure>
</t>
<t>
Dynamic table (after decoding): empty.
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
password: secret]]></artwork>
</figure>
</t>
</section>
<section title="Indexed Header Field">
<t>
The header field representation uses an indexed header field, from
the static table.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
82 | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET]]></artwork>
</figure>
</t>
<t>
Dynamic table (after decoding): empty.
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="request.examples.without.huffman.coding" title="Request Examples without Huffman Coding">
<t>
This section shows several consecutive header lists, corresponding to
HTTP requests, on the same connection.
</t>
<section title="First Request">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
8286 8441 0f77 7777 2e65 7861 6d70 6c65 | ...A.www.example
2e63 6f6d | .com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
41 | == Literal indexed ==
| Indexed name (idx = 1)
| :authority
0f | Literal value (len = 15)
7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
| -> :authority: www.example\
| .com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 57) :authority: www.example.com
Table size: 57]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Request">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
8286 84be 5808 6e6f 2d63 6163 6865 | ....X.no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
be | == Indexed - Add ==
| idx = 62
| -> :authority: www.example\
| .com
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | no-cache
| -> cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
Table size: 110]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache]]></artwork>
</figure>
</t>
</section>
<section title="Third Request">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
8287 85bf 400a 6375 7374 6f6d 2d6b 6579 | ....@.custom-key
0c63 7573 746f 6d2d 7661 6c75 65 | .custom-value]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: https
85 | == Indexed - Add ==
| idx = 5
| -> :path: /index.html
bf | == Indexed - Add ==
| idx = 63
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0c | Literal value (len = 12)
6375 7374 6f6d 2d76 616c 7565 | custom-value
| -> custom-key: custom-valu\
| e]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com
Table size: 164]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="request.examples.with.huffman.coding" title="Request Examples with Huffman Coding">
<t>
This section shows the same examples as the previous section, but using
Huffman encoding for the literal values.
</t>
<section title="First Request">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
8286 8441 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...A......:k....
ff | .]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
41 | == Literal indexed ==
| Indexed name (idx = 1)
| :authority
8c | Literal value (len = 12)
| Huffman encoded:
f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k.....
| Decoded:
| www.example.com
| -> :authority: www.example\
| .com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 57) :authority: www.example.com
Table size: 57]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Request">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
8286 84be 5886 a8eb 1064 9cbf | ....X....d..]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
be | == Indexed - Add ==
| idx = 62
| -> :authority: www.example\
| .com
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
86 | Literal value (len = 6)
| Huffman encoded:
a8eb 1064 9cbf | ...d..
| Decoded:
| no-cache
| -> cache-control: no-cache]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
Table size: 110]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: http
:path: /
:authority: www.example.com
cache-control: no-cache]]></artwork>
</figure>
</t>
</section>
<section title="Third Request">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
8287 85bf 4088 25a8 49e9 5ba9 7d7f 8925 | ....@.%.I.[.}..%
a849 e95b b8e8 b4bf | .I.[....]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: https
85 | == Indexed - Add ==
| idx = 5
| -> :path: /index.html
bf | == Indexed - Add ==
| idx = 63
| -> :authority: www.example\
| .com
40 | == Literal indexed ==
88 | Literal name (len = 8)
| Huffman encoded:
25a8 49e9 5ba9 7d7f | %.I.[.}.
| Decoded:
| custom-key
89 | Literal value (len = 9)
| Huffman encoded:
25a8 49e9 5bb8 e8b4 bf | %.I.[....
| Decoded:
| custom-value
| -> custom-key: custom-valu\
| e]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com
Table size: 164]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:method: GET
:scheme: https
:path: /index.html
:authority: www.example.com
custom-key: custom-value]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="response.examples.without.huffman.coding" title="Response Examples without Huffman Coding">
<t>
This section shows several consecutive header lists, corresponding to
HTTP responses, on the same connection. The HTTP/2 setting parameter
SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing
some evictions to occur.
</t>
<section title="First Response">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
4803 3330 3258 0770 7269 7661 7465 611d | H.302X.privatea.
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 546e 1768 | 20:13:21 GMTn.h
7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
6c65 2e63 6f6d | le.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 32 | 302
| -> :status: 302
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
6e | == Literal indexed ==
| Indexed name (idx = 46)
| location
17 | Literal value (len = 23)
6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
706c 652e 636f 6d | ple.com
| -> location: https://www.e\
| xample.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Response">
<t>
The (":status", "302") header field is evicted from the dynamic table
to free space to allow adding the (":status", "307") header field.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
4803 3330 37c1 c0bf | H.307...]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 37 | 307
| - evict: :status: 302
| -> :status: 307
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
c0 | == Indexed - Add ==
| idx = 64
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location: https://www.e\
| xample.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 42) :status: 307
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Third Response">
<t>
Several header fields are evicted from the dynamic table during the
processing of this header list.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com
content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
88c1 611d 4d6f 6e2c 2032 3120 4f63 7420 | ..a.Mon, 21 Oct
3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
54c0 5a04 677a 6970 7738 666f 6f3d 4153 | T.Z.gzipw8foo=AS
444a 4b48 514b 425a 584f 5157 454f 5049 | DJKHQKBZXOQWEOPI
5541 5851 5745 4f49 553b 206d 6178 2d61 | UAXQWEOIU; max-a
6765 3d33 3630 303b 2076 6572 7369 6f6e | ge=3600; version
3d31 | =1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
88 | == Indexed - Add ==
| idx = 8
| -> :status: 200
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location: https://www.e\
| xample.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
04 | Literal value (len = 4)
677a 6970 | gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
77 | == Literal indexed ==
| Indexed name (idx = 55)
| set-cookie
38 | Literal value (len = 56)
666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
6572 7369 6f6e 3d31 | ersion=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com
content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="response.examples.with.huffman.coding" title="Response Examples with Huffman Coding">
<t>
This section shows the same examples as the previous section, but using
Huffman encoding for the literal values. The HTTP/2 setting parameter
SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing
some evictions to occur. The eviction mechanism uses the length of the
decoded literal values, so the same evictions occurs as in the previous
section.
</t>
<section title="First Response">
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
4882 6402 5885 aec3 771a 4b61 96d0 7abe | H.d.X...w.Ka..z.
9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f...
2d1b ff6e 919d 29ad 1718 63c7 8f0b 97c8 | -..n..)...c.....
e9ae 82ae 43d3 | ....C.]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 2)
| Huffman encoded:
6402 | d.
| Decoded:
| 302
| -> :status: 302
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
85 | Literal value (len = 5)
| Huffman encoded:
aec3 771a 4b | ..w.K
| Decoded:
| private
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
96 | Literal value (len = 22)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e082 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:21 \
| GMT
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
6e | == Literal indexed ==
| Indexed name (idx = 46)
| location
91 | Literal value (len = 17)
| Huffman encoded:
9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
d3 | .
| Decoded:
| https://www.example.com
| -> location: https://www.e\
| xample.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:status: 302
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Second Response">
<t>
The (":status", "302") header field is evicted from the dynamic table
to free space to allow adding the (":status", "307") header field.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
4883 640e ffc1 c0bf | H.d.....]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
83 | Literal value (len = 3)
| Huffman encoded:
640e ff | d..
| Decoded:
| 307
| - evict: :status: 302
| -> :status: 307
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
c0 | == Indexed - Add ==
| idx = 64
| -> date: Mon, 21 Oct 2013 \
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location: https://www.e\
| xample.com]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 42) :status: 307
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:status: 307
cache-control: private
date: Mon, 21 Oct 2013 20:13:21 GMT
location: https://www.example.com]]></artwork>
</figure>
</t>
</section>
<section title="Third Response">
<t>
Several header fields are evicted from the dynamic table during the
processing of this header list.
</t>
<t>
<figure>
<preamble>Header list to encode:</preamble>
<artwork><![CDATA[
:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com
content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Hex dump of encoded data:</preamble>
<artwork><![CDATA[
88c1 6196 d07a be94 1054 d444 a820 0595 | ..a..z...T.D. ..
040b 8166 e084 a62d 1bff c05a 839b d9ab | ...f...-...Z....
77ad 94e7 821d d7f2 e6c7 b335 dfdf cd5b | w..........5...[
3960 d5af 2708 7f36 72c1 ab27 0fb5 291f | 9`..'..6r..'..).
9587 3160 65c0 03ed 4ee5 b106 3d50 07 | ..1`e...N...=P.]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoding process:</preamble>
<artwork><![CDATA[
88 | == Indexed - Add ==
| idx = 8
| -> :status: 200
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
96 | Literal value (len = 22)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e084 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:22 \
| GMT
| - evict: cache-control: pr\
| ivate
| -> date: Mon, 21 Oct 2013 \
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location: https://www.e\
| xample.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
83 | Literal value (len = 3)
| Huffman encoded:
9bd9 ab | ...
| Decoded:
| gzip
| - evict: date: Mon, 21 Oct\
| 2013 20:13:21 GMT
| -> content-encoding: gzip
77 | == Literal indexed ==
| Indexed name (idx = 55)
| set-cookie
ad | Literal value (len = 45)
| Huffman encoded:
94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P.
| Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ\
| WEOIU; max-age=3600; versi\
| on=1
| - evict: location: https:/\
| /www.example.com
| - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ\
| KBZXOQWEOPIUAXQWEOIU; ma\
| x-age=3600; version=1]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Dynamic Table (after decoding):</preamble>
<artwork><![CDATA[
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215]]></artwork>
</figure>
</t>
<t>
<figure>
<preamble>Decoded header list:</preamble>
<artwork><![CDATA[
:status: 200
cache-control: private
date: Mon, 21 Oct 2013 20:13:22 GMT
location: https://www.example.com
content-encoding: gzip
set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1]]></artwork>
</figure>
</t>
</section>
</section>
<!-- example-end -->
</section>
<section title="Change Log (to be removed by RFC Editor before publication)">
<section anchor="changes.since.draft-ietf-httpbis-header-compression-10" title="Since draft-ietf-httpbis-header-compression-10">
<t><list style="symbols">
<t>
Editorial corrections for taking into account IETF LC
comments.
<list style="symbols">
<t>
Added links to security sections.
</t>
<t>
Made spec more independent of HTTP/2.
</t>
<t>
Expanded security section about never indexed
literal usage.
</t>
</list>
</t>
<t>
Removed most usages of 'name-value pair' instead of
header field.
</t>
<t>
Changed 'header table' to 'header field table'.
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-09" title="Since draft-ietf-httpbis-header-compression-09">
<t><list style="symbols">
<t>
Renamed header table to dynamic table.
</t>
<t>
Updated integer representation.
</t>
<t>
Editorial corrections.
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-08" title="Since draft-ietf-httpbis-header-compression-08">
<t><list style="symbols">
<t>
Removed the reference set.
</t>
<t>
Removed header emission.
</t>
<t>
Explicit handling of several SETTINGS_HEADER_TABLE_SIZE
parameter changes.
</t>
<t>
Changed header set to header list, and forced ordering.
</t>
<t>
Updated examples.
</t>
<t>
Exchanged header and static table positions.
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-07" title="Since draft-ietf-httpbis-header-compression-07">
<t><list style="symbols">
<t>
Removed old text on index value of 0.
</t>
<t>
Added clarification for signalling of maximum table size
after a SETTINGS_HEADER_TABLE_SIZE update.
</t>
<t>
Rewrote security considerations.
</t>
<t>
Many editorial clarifications or improvements.
</t>
<t>
Added convention section.
</t>
<t>
Reworked document's outline.
</t>
<t>
Updated static table. Entry 16 has now "gzip, deflate"
for value.
</t>
<t>
Updated Huffman table, using data set provided by
Google.
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-06" title="Since draft-ietf-httpbis-header-compression-06">
<t><list style="symbols">
<t>
Updated format to include literal headers that must
never be compressed.
</t>
<t>
Updated security considerations.
</t>
<t>
Moved integer encoding examples to the appendix.
</t>
<t>
Updated Huffman table.
</t>
<t>
Updated static header table (adding and removing status
values).
</t>
<t>
Updated examples.
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-05" title="Since draft-ietf-httpbis-header-compression-05">
<t><list style="symbols">
<t>
Regenerated examples.
</t>
<t>
Only one Huffman table for requests and responses.
</t>
<t>
Added maximum size for dynamic table, independent of
SETTINGS_HEADER_TABLE_SIZE.
</t>
<t>
Added pseudo-code for integer decoding.
</t>
<t>
Improved examples (removing unnecessary removals).
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-04" title="Since draft-ietf-httpbis-header-compression-04">
<t><list style="symbols">
<t>
Updated examples: take into account changes in the spec,
and show more features.
</t>
<t>
Use 'octet' everywhere instead of having both 'byte' and
'octet'.
</t>
<t>
Added reference set emptying.
</t>
<t>
Editorial changes and clarifications.
</t>
<t>
Added "host" header to the static table.
</t>
<t>
Ordering for list of values (either NULL- or
comma-separated).
</t>
</list></t>
</section>
<section anchor="changes.since.draft-ietf-httpbis-header-compression-03" title="Since draft-ietf-httpbis-header-compression-03">
<t><list style="symbols">
<t>
A large number of editorial changes; changed the
description of evicting/adding new entries.
</t>
<t>
Removed substitution indexing
</t>
<t>
Changed 'initial headers' to 'static headers', as per
issue #258
</t>
<t>
Merged 'request' and 'response' static headers, as per
issue #259
</t>
<t>
Changed text to indicate that new headers are added at
index 0 and expire from the largest index, as per issue
#233
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-02">
<t><list style="symbols">
<t>
Corrected error in integer encoding pseudocode.
</t>
</list></t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-01">
<t>
<list style="symbols">
<t>
Refactored of Header Encoding Section: split
definitions and processing rule.
</t>
<t>
Backward incompatible change: Updated reference set
management as per issue #214. This changes how the
interaction between the reference set and eviction
works. This also changes the working of the
reference set in some specific cases.
</t>
<t>
Backward incompatible change: modified initial
header list, as per issue #188.
</t>
<t>
Added example of 32 octets entry structure (issue
#191).
</t>
<t>
Added Header Set Completion section. Reflowed some
text. Clarified some writing which was akward.
Added text about duplicate header entry encoding.
Clarified some language w.r.t Header Set. Changed
x-my-header to mynewheader. Added text in the
HeaderEmission section indicating that the
application may also be able to free up memory more
quickly. Added information in Security
Considerations section.
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-header-compression-00">
<t>
<list>
<t>Fixed bug/omission in integer representation
algorithm.</t>
<t>Changed the document title.</t>
<t>Header matching text rewritten.</t>
<t>Changed the definition of header emission.</t>
<t>Changed the name of the setting which dictates how
much memory the compression context should use.</t>
<t>Removed "specific use cases" section</t>
<t>Corrected erroneous statement about what index can be
contained in one octet</t>
<t>Added descriptions of opcodes</t>
<t>Removed security claims from introduction.</t>
</list>
</t>
</section>
</section>
</back>
</rfc><!--
vim:et:tw=80:sw=4:
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